Conventionally, there is known a document reader in which a plurality of line sensors are arranged in a zigzag manner in a copier or the like, whereby a first row and a second row of the line sensors extending in a main scanning direction are formed, a document is read with the first row of line sensors in advance, and subsequent to the first row, the document is read with the second row of line sensors shifted in a sub-scanning direction with respect to the first row (e.g., see JP 2004-214834 A and JP 2004-215011 A).
Further, there is disclosed a technique of correcting shading so as to enhance a dynamic range of output data of each photoelectric conversion element by suppressing the influence of a variation in characteristics of a number of photoelectric conversion elements arranged in the main scanning direction of the line sensors (e.g., see JP 5-122527 A).
The shading correction for each photoelectric conversion element is performed using the following expression.Number of gradation levels×(Document read value−Black reference read value)/(White reference read value−Black reference read value)In this expression, the white reference read value represents read data obtained by reading a white reference plate placed apart on an opposite side of the line sensors with respect to a transparent document support plate with each photoelectric conversion element of the line sensors under illumination. The black reference read value represents read data obtained by reading the white reference plate respectively with each photoelectric conversion element of the line sensors under the condition that illumination is off. The document read value represents read data obtained by reading a document on a document support respectively with each photoelectric conversion element of the line sensors under illumination. The number of gradation levels is represented by a gray gradation in which each read data (output voltage value) is generally associated with digital data in 255 stages.
In the document reader for reading a document using line sensors arranged in the zigzag manner, it is very difficult to arrange a plurality of line sensors with the distances to the document support plate being exactly the same. The variation in distances is caused by the variation in a size tolerance of μ order for each line sensor, the size tolerance of the thickness of a base and a document support plate on which a plurality of line sensors are set, and the distortion thereof.
If there is a variation in distances of each line sensor with respect to the document support plate, for example, as represented by a case where a document whose image all over the surface thereof is previously known to be a gray gradation is read with each line sensor, it is found that a document read value of a photoelectric conversion element read from each line sensor that divides and scans this document in a main scanning direction varies. Therefore, for example, in an image forming device such as a copier in which an image is formed based on a document read value read by a photoelectric conversion element of each line sensor, gradation occurs for each image region corresponding to a read area of the line sensors arranged in the zigzag manner, which is not preferable in terms of the quality of an image.
However, in any of JP 2004-214834 A and JP 2004-215011 A, there is no description regarding the problem caused by the variation in distance of each line sensor with respect to the document support plate and the countermeasure against the problem. Further, even if the shading correction described in Patent Document 3 is performed, the variation in a document read value read from the photoelectric conversion element of each line sensor cannot be reduced.
An object of the present invention is to obtain a document reader capable of suppressing the variation in document read values read from the photoelectric conversion element of each line sensor irrespective of the variation in distance of each line sensor with respect to a document support plate.